1. Field of the Invention
The present invention is useful in time-of-flight mass spectrometry (TOFMS), a method for qualitative and quantitative chemical analysis. Many TOFMS work with counting techniques, in which case the dynamic range of the analysis is strongly limited by the measuring time and the cycle repetition rate. This invention describes a detection method to increase the dynamic range of elemental-, isotopic-, or molecular analysis with counting techniques.
2. Description of the Prior Art
Anode: The part of a particle detector, which receives the electrons from the electron multiplier.
Anode Fraction: The fraction of the total amount of particles, which is detected by a specific anode.
Single Signal: The signal pulse produced by a detector when a single particle hits the detector. A counting electronics counts the single signals and their arrival.
Signal: A superposition of single signals, caused by particles of one specie hitting the detector within a very short time.
Time-of-flight mass spectrometers (TOFMS, see FIG. 1) allow the acquisition of wide-range mass spectra at high speeds because all masses are recorded simultaneously. Most TOFMS work in a cyclic mode. In each cycle, a certain number of particles (up to several thousand) are extracted and traverse a flight section towards a detector. Each particle""s time-of-flight is recorded to deliver information about its mass. Thus, in each cycle, a complete time spectrum is recorded and added to a histogram. The repetition rate of this cycle is commonly in the range of 1 to 50 kHz.
If several particles of one specie are extracted in one cycle, these particles will arrive at the detector within a very short time period (as short as 1 nanosecond). When using an analog detection scheme (transient recorder, oscilloscope) this does not cause a problem because these detection schemes deliver a signal which is proportional to the number of particles arriving within a certain sampling time. However, when a counting detection scheme is used (time-to-digital converter, TDC), the electronics cannot distinguish two or more particles of the same specie arriving simultaneously at the detector. Additionally, most TDCs have dead times (typically 20 nanoseconds), which prevent the detection of more than one particle or each mass in one extraction cycle.
For example, when analyzing an air sample with 12 particles per cycle, there will be approximately ten nitrogen molecules (80% N2 in air, mass=28 amu) per extraction cycle. These ten N2 particles will hit the detector within 2 nanoseconds (in a TOFMS of good resolving power). Even a fast TDC with only 0.5 nanoseconds timing resolution and no deadtime will not be able to detect all these particles because only one signal can be recorded each 0.5 nanoseconds. The detection system gets saturated at this intense peak. FIG. 2 shows these ten particles 5 of mass 28 amu impinging a detector of prior art. The TDC will register only the first of all these ten particles. Therefore peaks for abundant specie (N2 and O2) are artificially small and are recorded too early because only the first particle is registered. This effect is termed xe2x80x9csaturation.xe2x80x9d FIG. 9 shows the effects of saturation on the spectrum peaks for N2 and O2. To give a better overview, three different scalings of the same spectrum are shown. The abundance should be 78% N2, 21% O2 and 1% Ar. As shown in FIG. 9, the N2 peak and the O2 peak are much too small compared to the Ar peak which is not saturated (top and bottom panel). Saturation is so strong that there are virtually no counts during the dead time of approximately 20 nanoseconds registered (middle panel).
In an attempt to prevent saturation, some prior art detectors use multiple anodes. An individual TDC channel records each anode. FIG. 3 shows a prior art detector with four anodes of equal size. This allows the identification of four times larger intensities compared with a single anode detector. However, even with four anodes, the detection of the ten N2 particles leads to saturation because there are more than two particles per anode on average 6 and 7.
With more anodes, saturation could in principle be avoided, but as each anode requires its own TDC channel, this solution becomes complex and expensive.
Instead of using multiple equal sized anodes, the present invention uses multiple anodes wherein each anode has a different anode fraction. By reducing anode fraction, saturation can be eliminated. One method for achieving a different anode fraction is through use of anodes of different sizes as shown in FIG. 4 at 46 and 47. The example in FIG. 4 uses two unequal size anodes with a size ratio of approximately 1:9. As a result, the small anode only detects one particle 8 per cycle, just on the edge of saturation for N2. Less abundant particles like Ar (1% abundance in air=0.12 particles per cycle) are primarily detected and evaluated from the big anode which gives low statistical errors. Thus, with 2 anodes of unequal size it is possible to increase the dynamic range by a factor often or more. A prior art detector with equal sized anodes would require ten anodes to obtain the same improvement. It should be apparent that the dynamic range can be increased either by decreasing the anode fraction of the small anode or by adding additional anodes with even lower anode fractions. It is also possible to achieve differing fractions and to make such fractions adjustable by applying electric fields to influence the paths of incoming ions as shown in FIGS. 6 and 7.